One of the known techniques employed in satellite communications systems is TIME DIVISION MULTIPLE ACCESS. In this arrangement, a plurality of geographically separated earth stations employ the full bandwidth of a transponder located on board a spacecraft, in time multiplex fashion so that at any one time, only a single station's transmissions are received at the trasponder, but over a period of time (the TDMA frame), transmissions from each of a plurality of stations pass through the transponder. Although this type of communication regime has associated with it a well-developed body of art, see Sekimoto U.S. Pat. No. 3,320,611; Golding U.S. Pat. No. 3,566,267; and Gabbard U.S. Pat. No. 3,922,496, typically the timing systems are operated in a closed loop fashion. That is, more particularly, each station desiring to transmit has a predetermined time slot in the frame in which its transmission bursts should be positioned. Each station individually monitors the position of its own burst in the frame and corrects for any misalignment. The corrections thus are effective to eliminate timing errors due not only to terrestrial equipment, but also serve to correct for timing errors produced as a result of spacecraft motion and the resulting range variations.
The prior art has also suggested the use of simpler earth stations in which closed loop operation is eliminated. In the prior suggestions for what is thus termed "open loop" TDMA systems, each station employs a fixed nominal factor to compensate for propagation delays which factor was based upon the geographic location of the station and therefore the nominal satellite propagation delays.
In order to take into account expected variations in range, a guard band of some minimum time is employed between bursts emanating from different stations. Guard bands are employed in both closed and open loop TDMA systems, although in the former systems, the guard band between bursts is of relatively small duration since it is only necessary to compensate for equipment generated timing errors. In the prior open loop TDMA system referred to, however, a minimum guard band was equal to twice the variations in propagation delay produced as a result of the largest range variation.
Since the total guard time throughout a frame is the time during which transmissions are not occurring at any station, it represents a measure of communication capacity not being employed and therefore desirably should be minimized. It will be appreciated by those skilled in the art that the relative importance of this wasted communication capacity increases as the baud rate of the transmissions increases. One technique typically employed to minimize the effect of guard time is to lengthen the frame since, as the frame is lengthened, the percentage of frame time devoted to guard time is reduced and thus, efficiency is increased. A difficulty with increasing the frame duration is that it necessarily increases the size of the burst memories required at each of the stations and also increases the delay encountered in communication between one station and another.
It is therefore one object of the present invention to provide an open loop TDMA communications system in which guard time requirements are reduced below that required for prior art open loop TDMA communications systems. It is another object of the present invention to provide, in an open loop TDMA communications system, transmit timing adjustments for at least a plurality of the stations, which timing adjustments are made based on a satellite position index signal. It is another object of the present invention to provide an open loop TDMA communications system in which a transponder is located on a spacecraft in quasi-synchronous earth orbit and in which a reference station transmits a spacecraft position index signal, receivable at a plurality of stations which is employed to make transmit timing adjustments. It is a further object of the present invention to provide an open loop communications system in which transmit timing adjustments are made at each of a plurality of geographically separated stations based on a spacecraft position index signal, which transmit timing adjustments are uniquely related to the station's geographical position.
These and other objects of the invention will become apparent in the course of the following description of preferred embodiments of the invention.